1. Field of the Invention
Aspects of the present invention relate to a hologram optical device, a compatible optical pickup having the hologram optical device, and an optical information storage medium system including the compatible optical pickup, and more particularly, to a hologram optical device capable of reproducing and/or recording data from/to information storage media having different thicknesses using light emitted from the same light source, a compatible optical pickup having the hologram optical device, and an optical information storage medium system including the compatible optical pickup.
2. Description of the Related Art
The size of a spot of laser light focused on an information storage medium (i.e., an optical disc) via an objective lens determines the recording capacity of data on an optical recoding and/or reproducing apparatus. Specifically, the size of the optical spot is determined by the wavelength λ of the laser light and numerical aperture (NA) of the objective lens according to the following Formula 1:Diameter of focused spot∝λ/NA  (1)
Thus, to reduce the size of the optical spot formed on an optical disc and thus achieve high density recording, it is necessary to employ a short wavelength light source (such as a blue laser) and an objective lens having a high NA.
According to the Blu-ray Disc (BD) standard, a single side of the BD has a storage capacity of about 25 GB. A recording/reproducing apparatus uses a light source having a wavelength of around 405 nm and an objective lens having an NA of 0.85 to record/reproduce data on the BD. The thickness of the BD is 0.1 mm. The thickness is defined as an interval from a light incident surface to an information storage surface, which also corresponds to the thickness of a protection layer.
Also, according to the high definition DVD (HD DVD) standard, the HD DVD has a storage capacity of about 15 GB. A recording/reproducing apparatus uses a light source having a wavelength of around 405 nm (the same as in the case of the BD) and an objective lens having an NA of 0.65 to record/reproduce data on the HD DVD. The thickness of the HD DVD is 0.6 mm. As with the BD, the thickness is defined as an interval from a light incident surface to an information storage surface, which also corresponds to the thickness of a substrate.
Due to the use of both the BD having a storage capacity of about 25 GB in a single side and the HD DVD having a storage capacity of about 15 GB in a single side, there is a need for a compatible apparatus that can use both types of optical discs in a single system. In this regard, a method of using two objective lenses appropriate for both types of optical discs has been suggested. However, in this case, since two objective lens units and corresponding optical parts are needed, the number of optical parts increases. Thus, the production cost increases. Furthermore, an optical axis adjustment between the objective lenses is difficult.
To address the above problems, a method of using one objective lens unit and reducing a spherical aberration using a hologram optical device has been considered. Japanese Patent Publication No. 08-062493 discloses a method of compatibly using CD based optical discs with a DVD light source by using a hologram lens. According to this method, in separating light, a 0th order diffractive light as a straight transmission beam forms a focus and a +1st order diffractive light as a divergent transmission beam forms another focus having a different focus length. In the above publication, a hologram lens diffracts an incident light beam in the form of a parallel beam to a 0th order diffractive light and a +1st order diffractive light. The 0th order diffractive light is incident on the objective lens in the form of a non-divergent (non-convergent) beam. The incident beam is used to record information on a relatively thin optical disc or reproduce recorded information therefrom. Also, the +1st order diffractive light in the form of diverging beam is used to record information on a relatively thick optical disc or reproduce recorded information therefrom. The optical spot formed by the 0th order diffractive light for recording and reproducing a DVD and the optical spot formed by the +1st order diffractive light for recording and reproducing a CD are formed on the same optical axis.
As described above, conventionally, the 0th order diffractive light and the +1st order diffractive light are respectively used for straightly transmission and divergent transmission so as to record and reproduce not only a DVD, but also a CD with a DVD light source. However, even when the hologram optical device diffracts the incident light to the 0th order light and the +1st order lights, the amount of light of each of the other orders is not completely zero. That is, the hologram optical device substantially diffracts a small amount of the incident light to other orders. Thus, the 0th order diffractive light reflected by the optical disc and incident on the hologram lens is diffracted again by the hologram lens to the 0th order light, the +1st order light, the −1st order light, and so forth. Here, the 0th order diffractive light is used to detect a DVD reproduction signal. That is, for DVD reproduction, the 0th order/0th order diffractive light is used as a signal light.
Likewise, the 1st order diffractive light that is reflected by the optical disc and incident on the hologram lens is diffracted again by the hologram lens to the 0th order light, the +1st order light, the −1st order light, and so forth. Here, the +1st order diffractive light is used for detecting a CD reproduction signal. That is, for CD reproduction, the +1st order/+1st order diffractive light is used as a signal light.
The signal light for the DVD reproduction uses the 0th order diffractive light as an incident light and the 0th order diffractive light reflected by the optical disc and incident on the hologram lens. The signal light for the CD reproduction uses the +1st order diffractive light as an incident light and the +1st order diffractive light reflected by the optical disc and incident on the hologram lens. When these signal lights are used for the compatible reproduction of the BD and HD DVD, the 0th order/0th order diffractive light can be used as the signal light for the BD reproduction and the 1st order/1st order diffractive light can be used as the signal light for the HD DVD reproduction.
However, when a general optical axis rotation symmetric hologram optical device such as the conventional hologram lens is used, the light spot for reproducing the BD and the light spot for reproducing the HD DVD exist on the same optical axis. Accordingly, the light detected by a photodetector after being reflected by the optical disc and passing through the hologram optical device includes noise due to other diffractive lights so that the reproduction signal is degraded. That is, when the hologram optical device uses the incident light of nth order diffractive light/the return light of nth order diffractive light as a signal light, the incident light of (n−1)th order diffractive light/the return light of (n+1)th order diffractive light, and the incident light of (n+1)th order diffractive light/the return light of (n−1)th order diffractive light are incident on the photodetector and acts as noise affecting the signal light. In this case, the incident light is light diffracted by the hologram optical device and emitted to the optical disc. The return light is light reflected by the optical disc, incident again on the hologram optical device to be diffracted, and proceeding toward the photodetector.
When a collimated parallel light from a light source is incident on a hologram optical device and a BD is employed, since the 0th order diffractive light from the hologram optical device is reflected by the BD and incident again on the hologram optical device using an optical path of the 0th order diffractive light, the 0th order diffractive light of the hologram optical device proceeds as a parallel light after the hologram optical device. Also, since the 1st order light from the hologram optical device is reflected by the BD and is incident again on the hologram optical device using an optical path of the −1st order diffractive light, eventually the −1st order diffractive light is incident on the hologram optical device and the light proceeds as a parallel light after the hologram optical device. Likewise, since the −1st order light from the hologram optical device is reflected by the optical disc and is incident again on the hologram optical device using an optical path of the 1st order diffractive light, the light proceeds as a parallel light after passing through the hologram optical device.
When the BD is employed, the 0th order/0th order diffractive light, the −1st order/1st order diffractive light, and the 1st order/−1st order diffractive light proceed using the same optical path after the hologram optical device. Accordingly, when the 0th order/0th order diffractive light is used as a signal light to reproduce the BD, the −1st order/1st order diffractive light and the 1st order/−1st order diffractive light act as noise.
Likewise, when the 1st order/1st order diffractive light is used as a signal light for the HD DVD, the 0th order/2nd order diffractive light and the 2nd order/0th order diffractive light act as noise. As it can be seen from FIG. 1, the sizes of light spots formed on the photodetector by the −1st order/1st order diffractive light and the 1st order/−1st order diffractive light and the 0th order/2nd order diffractive light and the 2nd order/0th order diffractive light are similar to that of the light spot formed by the signal light. As a result, a reproduction signal is degraded. FIG. 1 shows spots of a signal light and a noise light received on a surface of a photodetector during the reproduction of a BD and HD DVD. Thus, to increase the quality of the BD and HD DVD reproduction signal, noise should be reduced by reducing the efficiency of the diffractive light generating the noise.
Meanwhile, since the 0th order/1st order and 1st order/0th order diffractive lights having a high diffraction efficiency form a large spot on the photodetector, the 0th order/1st order and 1st order/0th order diffractive lights do not have much of an affect on the reproduction signal.
However, for an optical disc using a differential push-pull (DPP) signal as a tracking servo signal (such as a WORM and a re-recordable optical disc), the −1st order/1st order diffractive light, the 1st order/−1st order diffractive light, the 0th order/2nd order diffractive light, and the 2nd order/0th order diffractive light are not the only lights that degrade a sub-push-pull (SPP) signal so that the DPP signal is degraded. The 0th order/1st order and 1st order/0th order diffractive lights also degrade the SPP signal so that the DPP signal is degraded. The reasons for this are as follows. Three light beams (a main light beam and two sub-light beams) divided by a grating for detection of the DPP signal form a light spot on the surface of an optical disc through an objective lens. In this case, the ratio of the amount of light between the main light beam and the sub-light beam is about 10:1. When a general optical axis rotation symmetric hologram optical device is used, the amount of light received in a sub-light receiving area for the detection of a sub-beam of the 1st order/0th order and 0th order/1st order diffractive lights of the main beam is large enough to affect the sub-push-pull signal.